The invention pertains to a process for manufacturing objects and in particular moulded components from fiber-reinforced thermoplastic materials by blending a thermoplastic resin with reinforcing fibers, plasticating the blend inside a screw-type extruder with the addition of heat, and extruding a plastic mass for moulding.
A process of this class is generally known. In this known process, a thermoplastic resin in granulate form, with particle size greater than 2 mm is mixed with reinforcing fibers and blended in a screw-type extruder. The resulting mass is kneaded and plasticated with the addition of heat from the exterior of the extruder. It is difficult to introduce the entire quantity of heat required to plasticate the thermoplastic resin from the exterior of the screwtype extruder since the walls of the extruder housing would become too hot. In this scenario, the low-viscosity thermoplastic material would collect along the inside surface of the extruder, adversely affecting the consistency of the material. Due to the poor heat conductivity of plastics, plastication solely by externally introduced heat to the interior of the extruder would also increase the time to plasticate the blend.
Consequently a combined heat generation technique is practiced. This technique involves a two step process. First, heat is introduced through the walls of the extruder housing into the interior of the extruder and to the thermoplastic resin mass located therein. Secondly, the action of the screw resulting from its rotation in the extruder housing introduces friction energy to the thermoplastic mass moved by the screw. The kinetic energy applied by the rotating screw to the mass is converted into thermal energy which can be used for additional heating of the blend. If the screw applies too much power to the blend, the reinforcing fibers blended with the thermoplastic will break and be shortened. The strength of the resulting product will therefore suffer.
In order to avoid this detrimental damage to the fibers, an intermediate step in semi-finished product manufacturing is practiced. Thus, for example, glass mats are manufactured and impregnated with a thermoplastic matrix or glass fibers are coated in an extrusion process with thermoplastic melt and the mass is subsequently granulated.
In such a process, it is possible to produce moulded parts with reinforcing fibers which are well maintained and well embedded but the additional effort required for manufacture of the intermediate product is considerable.
Thus, the object of the present invention is to indicate a process for manufacturing objects from fiber-reinforced thermoplastic resins in the known class, in accordance with which the parts moulded from long-fiber-reinforced thermoplastic resins can be manufactured more simply.
To attain this object, it is proposed that in the generic process the thermoplastic resin be in powder form, be blended with the reinforcing fibers and fed to the screw-type extruder. It is preferable that the thermoplastic powder be fed to the screw-type extruder at a mean particle size of less than 1 mm and more preferably less than 0.5 mm.
The object is achieved with the measures in accordance with the invention. As a result of the fineness of the thermoplastic materials mixed with the reinforcing fibers and fed to the screw-type extruder, the plastication of the blend is achieved at lower levels of power being applied by the effect of the rotating screw on the thermoplastic blend located in the extruder, i.e. with less friction energy. The result is that the long reinforcing fibers in the resulting plasticated mass generally retain their length. Consequently, their reinforcing effect is generally unaffected.
Due to the small grain size of the thermoplastic resin powder, early distribution with the reinforcing fibers is achieved so that a larger contact surface area is available between the fibers and the matrix from the very start of the heating process. In this way, improved fiber wetting with gentle heating is achieved from the commencement of the process. In addition, a large amount of friction energy is not required. Consequently, the fibers experience minimal damage. The parts thus manufactured achieve considerably better mechanical properties, and in particular demonstrate a generally high impact resistance.
Glass fibers are most frequently used as the reinforcing fibers. Alternatively, other reinforcing fibers such as natural fibers, e.g. flax, mineral-based fibers or even synthetic fibers are used. Fibers can be fed to the screw-type extruder pre-blended with the thermoplastic powder. Alternatively, a metered feed of the components can be effected separately wherein blending takes place inside the extruder.
In a preferred embodiment, the thermoplastic powder is a non-compounded raw polymer which is already obtained in the polymerization process at the appropriate fine grain size. Additives such as stabilizers, flame suppressants, and the like can be added in granulate or powder form, to a quantity of about 10 per cent by mass.
The invention is elucidated in greater detail using the following example.